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The two tails of Hurricane Isaac: Part 1

Hurricane’s effects observed in the U.S. Midwest and in the Gulf of Mexico

Hurricane Isaac provided rain to areas of the Midwest suffering from drought in August-September 2012.

Introduction

Prior to the incursion of “Superstorm Sandy” on the East Coast of the United States in October 2012, another hurricane – Isaac – captured headlines and posed a serious danger to the northern coast of the Gulf of Mexico in September. Isaac threatened the same region that had been devastated by Hurricane Katrina in 2005, exactly seven years earlier. Isaac made landfall as a Category 1 hurricane almost exactly seven years after Category 3 Katrina. Isaac actually made two official landfalls – first on Plaquemines Parish, the arm of land that extends southward from New Orleans as part of the Mississippi River delta, on August 28, and then another landfall on the southern Gulf Coast of Louisiana on the morning of August 29.

Part 1: The tale of Isaac’s tail on land

Even though the sustained winds of Isaac were on the lower end of Category 1, one of the main reasons causing significant problems was that the forward progress of the storm slowed down at landfall. Isaac thus had more time to cause extended storm surge, flooding, and daily rainfall amounts exceeding the 10-day climatological equivalent for this time of year. Particularly strong impacts were suffered by low-lying Plaquemines Parish, and coastal regions of Mississippi, Alabama, and Louisiana (Figure 1). When it finally lumbered inland, Isaac moved northward over Arkansas and then turned to the east over Missouri, southern Illinois, Kentucky, and Indiana. Figure 1 shows 14-day precipitation totals as measured by the Tropical Rainfall Measuring Mission (TRMM), during the period of August 25 – September 7, 2012, in millimeters per square meter.

Figure 1. 14-day precipitation totals as measured by TRMM for the period of August 25 – September 7, 2012. The officially registered track of Isaac is shown as “+” symbols, where each symbol represents the position of the center of Isaac at 3-hour intervals. Note how the storm slowed as it encountered the coast, dropping substantial rain on Louisiana, southern Mississippi, and far southern Alabama. The color bar cutoff at 30 mm is consistent with the approximate 14-day climatological rainfall amounts for Texas and surrounding inland areas. The intervals on the color bar are in approximately monthly amounts for those regions. (Click the image to view it larger.)

For comparison, Figure 2 displays the North American Land Data Assimilation System (NLDAS) Primary Forcing data for the same period, recalculated in millimeters of rain per day (mm/day). (NLDAS will be discussed in subsequent paragraphs.)

Figure 2. NLDAS Primary Forcing precipitation data for the period August 27-September 4, 2013, giving model perspective on how the rain persisted and changed along the path. (Click on the image to view it larger.)

Figure 3 shows an animation of Isaac’s path from the Florida Keys to the Mississippi Delta and then inland, using daily TRMM data. In the movie, it can be seen how the storm slows down on August 28-30 as it encounters northerly flow from a high pressure system that will be shown in Figure 8.

Figure 3. Animation of daily TRMM rainfall for Hurricane Isaac, showing the movement of the storm.

Although Isaac’s rainfall delivered some relief to Arkansas and Missouri area, it wasn’t enough to break from the grasp of an extended drought, part of a larger pattern of long-term drought and warm conditions that have persisted in the region for several years. Moderate to severe drought conditions persisted over large swaths of Texas, New Mexico, Oklahoma, Arkansas, even after the storm. (Click on the image to view it larger.) Alternative versions of this animation are provided to allow pausing and examination of individual frames:

Although Isaac’s rainfall delivered some relief to Arkansas and Missouri area, it wasn’t enough to break from the grasp of an extended drought, part of a larger pattern of long-term drought and warm conditions that have persisted in the region for several years. Moderate to severe drought conditions persisted over large swaths of Texas, New Mexico, Oklahoma, Arkansas, even after the storm.

Thus, the ground in this region was very dry before Isaac's rains – another way to express that, using a hydrological science term, is that the soil moisture was very low.

NLDAS employs models that are based on the input of meteorological data to provide many different kinds of hydrological data, which can be compared to satellite measurements of the same data variables. Many of the NLDAS data variables can be analyzed using the National Aeronautics and Space Administration (NASA) Giovanni data analysis system. One of these variables is soil moisture, expressed as kilograms of water per square meter of ground surface (kg/m2). The models provide data for several different depths below the ground surface. Figures 4 and 5 show comparisons of the soil moisture in the 1-10 centimeter layer and the 10-40 centimeter layer below the ground surface, respectively, for August 26 (before Isaac affected this region) and for September 2, after Isaac had deposited its volume of rain over this region. Both of the surface layers show markedly increased levels of soil moisture, especially in Arkansas, Missouri, and southern Illinois. This path of elevated soil moisture is the northern “tail” of Isaac’s path over the United States.

Figure 4. Soil moisture in the 0-10 cm layer before the passage of Isaac (left) and after the passage of Isaac (right). Soil moisture increased significantly over much of Arkansas, Missouri, and southern Illinois, but this provided only minimal relief to long-term drought conditions. (Click any image to view larger.)

Figure 5. Soil moisture in the 10-40 cm layer before the passage of Isaac (left) and after the passage of Isaac (right). Soil moisture increased significantly over much of Arkansas, Missouri, and southern Illinois.

It is interesting to note that there is not a direct correspondence of where Isaac’s inland rainfall fell and where the greatest changes in soil moisture were observed. These observations could be due to two factors; one, the areas where the soil was driest would have a large change in soil moisture from even a small amount of rain; and two, local topography can influence how much rain stays in the soil and how much runs off the land to streams and rivers. An increased slope can lead to more runoff than absorption.